How to scale portrait in Character(Script)

Im a new user in unity but im very interesting to make game with unity...
im currently make game with fungus tools,and when i add some portrait(for character script) it become much bigger than i expected (the size in game)..
i dont know how to scale/resize it ,or perharps should i import a lower resolution in my assets ?
sorry for my broken english and thx for answering !